How Customized Rope Coatings Extend the Lives of Maritime Cordage
Date: 27th September 2018
Marine cordage is not only expensive but is of critical importance to keeping vessels properly moored or controlled during towing. These are typical applications in which fiber-to-fiber abrasion is difficult to recognize and monitor. Yet the stress of the heavy-duty use to which such cordage is subjected causes costly wear and, occasionally, catastrophic failure. Fortunately, there is a way to extend cordage service life at a remarkably reasonable price: specialized fluoropolymer coatings.
These coatings extend the rope’s life by reducing the internal fiber-to-fiber degradation due to abrasion under severe pressure. As a result, winch ropes or ropes repeatedly riding over pulleys and sheaves can realize a significant increase in usable life. Given the cost of high-performance ropes, made of Ultra High Molecular Weight Polyethylene (HMPE), Liquid Crystal Polymer (LCP), or Aramid fibers, extending their lifespans represents a considerable saving.
In the design and manufacture of commercial marine cordage, calculating the ratio between a rope’s working load and its minimum breaking strength is a major consideration, according to the Cordage Institute (CI), an international association of rope, twine and related manufacturers, their suppliers and affiliated industries. The CI attempts to identify tests and standards that replicate the stresses and conditions the product will experience in use. For example, the Cyclical Bend Over Sheave Test imparts cyclic loads on the test rope over a pulley, simulating the stresses a rope would undergo in the field.
Failure Due To External and Internal Abrasion
“External abrasion is the obvious wear indicator that everyone can see, where the rope starts to fray, then fail,” says Spencer Siegel, global business manager – technical textiles, of Whitford Corporation, which manufactures coatings and treatments for rope and cordage. “Internal abrasion is not easily seen, but is often the cause of catastrophic failure. When a rope is pulled back and forth repeatedly over an edge or a sheave, it is going around a tight turn. Consequently, the outer fibers have to travel a longer distance than the inner fibers, so there is unavoidable shear. Add a load and the shear stresses increase. With that shear, the fibers rub against each other and get very hot at the interfaces. This weakens and wears the individual fibers and, eventually, the integrity of the rope is compromised.”
Whitford made the move into coatings for cordage began in the mid-1990s. “We were providing coatings for a rubber company, which makes timing belts,” says Andrew Melville, who was in charge of Whitford’s textile coatings business at the time and now heads the company’s South American operations. “Our coatings enabled the timing belts to last longer. As a result, the belt manufacturers could start issuing guarantees on their lifespans.”
When a timing belt ruptures in a vehicle engine, it involves a lot of work and expense to put things right. Essentially, a timing belt is made up of layers of rubber and a woven textile for strengthening. The coated textile and rubber are bound together in layers under pressure and high heat to create a composite material.
“While we assumed that our coating was being applied to the rubber to provide it with an increased lifespan,” says Jessica Mannino, technical sales representative – ropes and textiles, “the coating actually was doing more for the textile component. The reason timing belts fail is that the reinforcing textile eventually wears away. When the belts are running at high speed, they flex. This puts force onto the textile reinforcing structure and eventually causes it to fracture. Our coating was helping the fibers lubricate each other, reducing the intra-wear within the woven textile. In the process, it was giving the textile better abrasion resistance and a longer lifespan.”
Relevance to the Maritime Industry
Where that became relevant to the maritime industry, according to Mannino, was with HMPE, LCP, or Aramid fiber ropes, by providing internal lubrication for the fibers.
Whitford’s EterniTex® coatings are custom-engineered for specific applications, attributes, and/or processes. In addition to ropes, they are also applied to other industrial textiles, such as belts and nets, to extend their service life. Because usage and subsequent failure modes of textiles vary depending upon their applications, EterniTex coatings are formulated for the specific use and environment in which the product must function. For example, the formula for extending the life of a commercial marine rope exposed to salt water and very high loads is very different from one engineered for a cargo net. In addition to low friction and resistance to internal abrasion, EterniTex coatings have been formulated to increase traction without increasing tackiness. This improves the handling and gripping ability especially of HMPE ropes, which are exceptionally strong, but slippery.
The coatings are typically applied using a dip process, followed by squeegeeing and drying processes. The final braided rope is passed through a bath of the coating, squeezed to control the desired pick-up weight, then either air dried or force dried at an elevated temperature.
“Whitford supplies customized coatings to customers who use them to meet specific requirements,” Spencer Siegel says. “Our EterniTex coatings protect and extend the working life and performance characteristics of highly engineered, mission-critical and expensive ropes.”
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